Continuity
or, using the substantive derivative:
For an incompressible fluid, is constant, hence the velocity field must be divergence-free:
Momentum
(Also known as the Navier-Stokes equation.)
or, using the substantive derivative:
where is a volumetric force (e.g. for gravity), and is the stress tensor.
The vector quantity is the shear stress. For a Newtonian incompressible fluid,
with being the (dynamic) viscosity.
For an inviscid fluid, the momentum equation becomes Euler's equation for ideal fluids: